LIFOS linker

;;Program var types:
;;Unrelocatable (NRP):
;;  Assembled to run only from $8100 in RAM
;;Fully relocatable (FRP):
;;  May be run from anywhere in RAM (uses relative references only)
;;DRP (dynamically relocatable):
;;  May be run from anywhere in RAM, but must first be dynamically linked by the OS
;;  to account for absolute data references.
;;
;;VAT entry spec:
;;  type | 1 [VtProg*]
;;  page | 1
;;  addr | 2
;;  name | 10
;;New header spec:
;;  size | 2        | Variable size (including header)
;;  version | 2     | Minimum OS version
;;==== Special entries valid only for DRPs ====
;;TDS: transient data segment- linked in after program code and initialized
;;     to all 0.
;;PDS: persistent data segment- is stored elsewhere and preserved across
;;     program invocations.  Linked in after the TDS.
;;==== Current DRP header version: 0       ====
;;  headerv | 1     | DRP header version (flags, too?)
;;  pdssize | 1     | Size of PDS    \Optional?  headerv could flag presence of
;;  pdssig  | 4     | PDS signature  |PDS and VDS, allowing them to be omitted.
;;  tdssize | 2     | Size of TDS    /
;;  data    | n     | Program code- main entry point
;;==== Linker Notes                        ====
;;The linker first calls allocVar at the lowest available location in user
;;memory, then allocates a chunk of memory after that for the VDS.  The PDS
;;file with matching signature will be allocated immediately following the VDS,
;;and a PDS file will be created first if it does not exist.
;;Finally, the linker creates the ULS (unlink segment) for its own use.
;;
;;Beginning at the main entry point, the reassembling linker traces program
;;flow from the main entry point, locating relink markers, likely a rarely-used
;;but legal instruction (`ld a,a', etc?) followed by relative address (offset
;;from 0x0000).  Various relink markers are modified to become absolute
;;references, perhaps `ld a,a' (0x7F) becomes `call' (0xCD), and the base link
;;address of this instance of the program is added to the immediately following
;;constant, resulting in a valid absolute reference.  The address of the relink
;;marker is then appended to the ULS so the reassembly can be undone when the
;;program exits.
;;
;;Possible table of relocation markers:
;; ld b,b       -> ld bc,nnnn
;; ld d,d       -> ld de,nnnn
;; ld h,h       -> ld hl,nnnn
;; ld c,c       -> ld bc,(nnnn)
;; ld e,e       -> ld de,(nnnn)
;; ld l,l       -> ld hl,(nnnn)
;; ld a,a       -> jp nnnn
;; ld (hl),(hl) -> call nnnn
;;
;;Example: [p+n] is n bytes from the program entry point
;; Program loaded at 0x81C2, with call to p+0x72F and reference to p+0x1024
;; (which, for demonstration, can be in the PDS) by HL immediately following.
;;Assembled/stored as:
;; ld (hl),(hl)
;; .dw 0x72F
;; ld h,h
;; .dw 0x1024
;;Linked as:
;; call 0x88F1
;; ld hl,0x91E6